The student is expected to compare characteristics of transverse waves, including electromagnetic waves and the electromagnetic spectrum, and characteristics and behaviors of longitudinal waves, including sound waves.
Waves can be categorized according to the direction the wave moves in relationship to the motion of the individual particles of the medium. Transverse waves propagate perpendicular to the displacement of the transmitting field or medium. Longitudinal waves propagate in the same direction as the displacement of the transmitting field or medium.
The electromagnetic spectrum consists of the range of all possible wave frequencies of electromagnetic radiation.
Mechanical waves require a medium to propagate through and travel relatively slowly. Electromagnetic radiation can travel through a vacuum and travels very fast.
Mechanical waves are produced by the vibration of particles in a medium, while the electromagnetic spectrum is produced by the movement of charged particles resulting in changes in electric and magnetic fields.
Examples of transverse waves include water waves and waves on a string. Examples of longitudinal waves include sound waves. The electromagnetic spectrum is considered to be only transverse.
Fluids allow the propagation of only longitudinal waves, such as sound. Both longitudinal and transverse waves can propagate through solids. The electromagnetic spectrum can travel through solids, liquids or gases, but is not propagated as a result of the disturbance of the medium.
Wave Motion
In physics, a wave is a disturbance or oscillation that travels through space and matter, accompanied by a transfer of energy. Wave motion transfers energy from one point to another. Waves that transfer energy through a vacuum and do not need a medium are electromagnetic waves. Waves that transfer energy through a medium are called mechanical waves. Mechanical waves and electromagnetic waves are different in many aspects.
Mechanical Waves, such as sound waves, ocean waves, and earthquake waves, transfer energy through a medium. Sound, for example, is produced by the vibration of particles in a medium, and move relatively slowly through the medium. The sound wave is a kind of mechanical wave, which propagates at the speed of 343.2 m/s at 20 degree centigrade in dry air. Although you might think this speed is quite fast, electromagnetic waves are much faster.
Electromagnetic waves are produced by the movement of charged particles resulting in changes in electric and magnetic fields. An electromagnetic wave consists of two waves that are oscillations of the electric and magnetic fields. An electromagnetic wave travels in a direction that is at right angles to the oscillation direction of both fields.
Speed: The entire electromagnetic spectrum of waves travel very fast through charged fields (the fastest in the universe). So electromagnetic waves are faster than mechanical waves. People use cell phones every day. A cell phone signal, which is a type of electromagnetic wave, travels nearly 300,000,000 m/s in the air. That’s why we can call someone else thousands of miles away in another continent instantly. The difference in wave speed is also the reason lightning (electromagnetic wave) is seen before the thunder (sound wave) is heard.
No Medium Required: The radiation in the electromagnetic spectrum can pass through solids, liquids or gases. It is not propagated as a result of the disturbance of the medium. So, the propagation of electromagnetic waves does not require a medium. It can even transmit in vacuum. Electromagnetic waves far from the Sun can travel about 93 million miles to the Earth through the vacuum of outer space.
The Electromagnetic Spectrum: The electromagnetic spectrum consists of the range of all possible wave frequencies of electromagnetic radiation including visible wavelengths called light and invisible wavelengths. Electromagnetic radiation with low frequency, long wavelengths are found in the microwave, radio, and infrared portion of the electromagnetic spectrum. High frequency, short wavelengths are found in the ultraviolet, x-rays, and gamma rays portion of the electromagnetic spectrum. Visible light occurs in the middle of those two extremes.
Wave Direction and Particle Motion
Waves can also be classified as transverse waves and longitudinal waves which are categorized according to the direction the wave moves in relationship to the motion of the individual particles of the medium. The medium also ‘selects’ which type of wave it can be transmitted through. In both types of waves, the individual particles will move up and down or back and forth, passing the energy as they collide with neighboring particles, but do not travel the length of the wave from the source to the final destination.
Transverse waves are propagated in a direction perpendicular to the displacement of the transmitting field or medium. Mechanical transverse waves cannot propagate in a fluid (gas or a liquid) because there is no mechanism for driving motion perpendicular to the propagation of the wave (sideways or left to right).
Characteristics of transverse waves include the highest point of the transverse wave called the crest and the trough, the lowest part of a transverse wave. The amplitude is the maximum distance that the particles of a wave’s medium vibrate from their rest position. It is half the vertical distance between the crest and the trough. Larger waves have more amplitude and carry more energy. A wavelength is the distance from any point on a wave to an identical point on the next wave.
Examples of transverse waves are certain mechanical waves like a ripple on a pond, a wave on a string, an audience wave, and secondary (S) seismic earthquake waves. Electromagnetic waves are actually 2 transverse waves: one an electrical field and the other a magnetic field.
Longitudinal waves are propagated in the same direction as the displacement of the transmitting field or medium. Longitudinal waves can travel through a medium in which the particles slip past each other.
Characteristics of longitudinal waves include regions of high and low density known as condensations and rarefactions that oscillate around local positions of equilibrium parallel to the path of energy transfer. Because the waves compress fluids, another term for a longitudinal wave is a compression wave. Fluids (air and liquids) transmit only longitudinal waves, such as sound.
Examples of longitudinal waves are: a slinky, ocean waves, primary (P) seismic earthquake waves, and sound waves in the air.
Solids can transmit both longitudinal and transverse waves. The waves of an earthquake contain both longitudinal and transverse waves. The Earth is solid, and thus it can transmit both. Fluids can only transmit longitudinal waves. Electromagnetic waves can travel through solids, liquids or gases, but is not propagated as a result of the disturbance of the medium.